The field of the invention is improved shrinkage compensating concrete. As is well known, traditional concrete tends to shrink as it dries or cures. This shrinkage occurs with loss of water as the concrete dries. The drying shrinkage creates tensile stresses in the concrete. Since concrete generally has low tensile strength, shrinkage stresses often cause cracking.
To avoid or reduce cracking caused by shrinkage, various expansive concretes have been used. See for example Klein, U.S. Pat. No. 3,251,701, Rice, U.S. Pat. No. 4,419,136, and Rice U.S. Pat. No. 5,846,316, each incorporated herein by reference. These and other shrinkage-compensating concretes include an expansive component or cement. The expansive cement generally is a hydraulic cement that itself includes an expansive component that expands during hydration. The expansive cement causes the concrete to expand slightly as it dries, which helps to offset or compensate for the shrinkage associated with drying. As a result, shrinkage and resulting tensile stresses in the concrete are reduced or eliminated, along with the cracking resulting from those stresses.
The tensile strength of concrete increases over time, using an expansive cement can also help reduce shrinkage cracking by reducing the tensile stresses, until the concrete acquires sufficient tensile strength to better withstand the tensile stresses without cracking. After the concrete has expanded, subsequent drying shrinkage will reduce the expansive stresses. Ideally though, a residual compression may remain in the concrete indefinitely, thereby eliminating shrinkage cracking.
Shrinkage compensating concrete conventionally requires a restraint element to prevent the concrete from over-expanding, which leads to cracking, crumbling and/or spalling. The restraint element may be external, such as other building structures, or temporary external construction plates or bars, such as described for example in Ytterberg U.S. Patent Application No. 2009/0071086. More often though, the restraint element is provided internally using steel rods, bars, mesh or fibers embedded into the shrinkage compensating concrete.
Since the advent of shrinking compensating concretes, for example beginning with Klein U.S. Pat. No. 3,251,701 as far back as 1961, or earlier, the industry and engineering convention has been that shrinkage compensating concretes must be restrained to achieve desired performance. Indeed, the relevant material standard, ASTM C 845 even defines shrinkage compensating concrete as a concrete that is internally restrained with resilient (e.g., steel) reinforcing and made with expansive cement with induces compressive stress in the concrete that approximately offsets tensile stresses that result from drying shrinkage. The minimum percentage of steel for restraint is 0.15% of the cross-sectional area. This is the restraint used in ASTM 878 Standard Test Method for Restrained Expansion of Shrinkage-Compensating Concrete. Correspondingly, various building codes specify that structures made with shrinkage compensating concrete must have a minimum amount of restraint, typically specified as a minimum amount of steel restraining rods. Thus, for over 50 years all known shrinkage compensating concrete structures have used added restraining elements.
An ideal shrinkage compensating concrete would have sufficient self-restraint to avoid cracking, without use of any steel bars, steel or non-metal fibers, or any other added restraining element. However, no such shrinkage compensating concrete has yet been realized.